1. Field of the Invention
The present invention relates to a method of decreasing the bulging of gable top cartons filled with liquid products. More particularly, the present invention relates to a method of increasing the air-tightness of the carton top seal by achieving a good seal, and eliminating heat activation pinhole damage below the top seal, which was acceptable in conventional containers, but which defeats the benefits of a good seal, with regard to air-tightness. The present invention creates the necessary conditions for formation of a partial vacuum in the headspace air, as the carton side walls start to bulge. The partial vacuum offsets the hydrostatic forces within the liquid, thereby decreasing the rate at which the carton bulges over a period of time.
2. Description of the Prior Art
Gable top cartons are widely used to contain liquids and are made from paperboard coated on their inner and outer surfaces with a thermoplastic coating such as low density polyethylene (LDPE). The coated paperboard is scored to define individual panels, and is folded along the score lines. The edges are sealed to form the carton.
Gable top cartons have a tendency to gradually change cross-sectional shape from square or rectangular cross-section to a rounded cross-section, otherwise known as bulging. Such bulging of gable top cartons is a common problem, especially when the shelf life of the carton exceeds fourteen days. Products packaged in gable top cartons can have shelf lives ranging up to 120 days.
The tendency of gable top cartons to bulge was previously believed to be caused by the physical properties of the paperboard and to some extent the quality of the top seals. It was believed that high top seal heat directed over an area covering and extending slightly beyond the area to be sealed provided a good top seal. Consequently, most efforts to improve bulge resistance of gable top cartons have been focused on improving the physical properties of the paperboard. For instance, foil laminated gable top cartons have been used for some time, and show greater resistance to bulging than non-foil cartons. The improved resistance to bulging was attributed to the improved physical properties resulting from foil lamination in the packaging. However, the concept of controlling bulge resistance of gable top cartons exclusively by improvements to the physical properties of the paperboard severely limits the success of controlling bulging.
While improvements to the physical properties of paperboard can improve bulge resistance, such improvements involve adding material or layers to the paperboard which increases costs in both labor, machinery and material.